The present invention relates to an optical head apparatus for recording and reproducing optical data recorded on an optical memory medium such as an optical disk, optical card and the like, a magnet-optic recording medium and the like.
An optical memory technology, which can develop a pit-shaped pattern in correspondence to signals in time series, has been practically used as a memory medium of high density and large capacity with the fields of application thereof extended from a digital audio disk to a data file.
The optical memory technology starts from the development of a memory medium and is completed by the development of an optical head apparatus. In particular, the reduction in size and cost and the improvement of performance and reliability of the optical head apparatus is indispensable to the spread of the optical memory technology and an optical head apparatus having an integrated source and photodetecting system as well as integrated electronic circuits has been developed.
FIGS. 1(a) and 1(b) show two examples of the optical system of a conventional integrated optical head apparatus. FIG. 1(c) is a detailed view of the optical system shown in FIG. 1(a), wherein the wavefront of divergent light from a source 100 using a semiconductor laser is divided into a main beam and two auxiliary beams at a first grating 101 and incident on a second grating 102 and a third grid 103 each having a semi-circular opening. A zero order diffracted wave component of the incident beam is focused on a memory medium 4 through a collimator 104 and an objective lens 105. Here, the distance d' between the spots of the main beam 107a and auxiliary beams 107b, 107c is designed to about 10 to 20 micrometers. These beams are reflected or diffracted by the pit surface of the memory medium 4, passes again through the objective lens 105 and the collimator 104, diffracted by the second and third gratings each arranged as a partial pattern of a holographic element, and received by a photodetecting system 108 split into 5 regions, whereby a focus is detected by a Foucault method and a tracking and an RF signal are detected by a 3-beam method. Note that the gratings 101, 102, and 103, which are formed on the opposite surfaces of the holographic element 109 can split the light into a plurality of wavefronts, and can be easily copied.
However, when a semiconductor laser is used as a source, in the optical head apparatus, the permissible fluctuation of a wavelength of the optical head apparatus is limited to about +10 nm even if other assembly errors are zero. Further, the tolerance of the relative positional error between the source 100 and the photodetecting system 108 is set to .DELTA.X.congruent.17 micrometers in the X-axial direction and .DELTA.Y.congruent.40 micrometers in the Y axial direction in the example shown in FIG. 1 for the reduction in size and the integration of the optical head apparatus, supposing that the objective lens has the focus length f=4.5 mm and the numerical aperture NA=0.45, and the collimate lens has the focus length f=21 mm and the numerical aperture N=0.11, and thus the assembly must be carried out with a pinpoint accuracy in a manufacturing process. In particular, in the case of an optical disk drive device provided with a recording/erasing system, it is difficult to ensure the reliability thereof in the circumference in which the device is used. This is because that a conventional Foucault method or astigmatic method is used as a focusing servo signal sensing system for the optical head apparatus, that is, this is because of the restriction resulting from the system (Foucault method) for forming a micro-spot (.about.10 micrometer dia.) on the surface of the photo-sensor or the system (astigmatism method) for sensing the balance of a distributed light quantity on the surface of a photo-sensor split into four sections. Whereas, there has been developed an optical head apparatus employing a spot size detection system (SSD) by which spots focused on two different focuses are differentially detected by linear photo-sensors interposed between the two focuses, as a focusing signal detection system which can allow a large amount of the relative positional error between a source 100 and a photodetecting system 3600 as shown in FIG. 1(b). (U.S. Pat. No. 4,929,823).
However, since this system obtains a tracking signal by differentially sensing a far field pattern of the tracking groove or pit in a memory medium 4, when a memory medium having only a pit train without having a groove, such as the so-called compact disk is reproduced, the tracking signal is unstably detected due to the offset of a servo signal caused by the inclination of the compact disk or the movement of an objective lens 3. Further, there remains a problem to realize a hologram element which can be easily blazed for restraining an unnecessary diffracted beam component as a hologram head arrangement for realizing a SSD system. In the arrangement shown in FIG. 1(b), the pattern of a third grating has a grating-shape having pitches of a few micrometers and further the pitches are set to 1 micrometer or less at the portion of the gratings where they intersect each other in order to make a distance h between the third grating and a source 100 to be a few millimeters, and thus the blazing thereof is very difficult.
Therefore, an object of the present invention is to provide an optical head apparatus wherein a large amount of the assembly error of a source and a photodetecting system is allowed when they are integrated, a stable servo signal with a restrained offset can be detected even from a memory medium having a shallow pit depth, and a diffraction optical element is provided which is easily blazed for restraining an unnecessary diffracted beam component. Further, another object of the present invention is to integrate an optical system as a whole to thereby further reduce the size of an optical head apparatus, lower the cost, and improve the mass-productivity and reliability thereof.
Here, a definition should be made, that is, the term "blazing" gives a meaning of giving optical characteristics to an optical element such that photoenergy is concentrated to a specified diffraction component.